The log evaluation of an expelling source rock is notoriously challenging because the kerogen and the generated oil coexist in the source rock. The Athel silicilyte is both an overpressured light-oil reservoir and a world-class source rock. In this unique geological setting found in the South Oman Salt Basin, reservoir characterisation is intimately associated with source rock evaluation.
In the absence of any direct geological analogue, the initial petrophysical model was calibrated against extensive core analyses. Porosity interpretation was then based exclusively on wireline bulk density measurements. The limitations of this approach only became apparent as attempts to reconcile data from various sources failed. While efforts aimed at obtaining representative core material and establishing reliable core analysis procedures, in-situ calibration of the petrophysical model was also actively pursued. NMR logging was primarily introduced to reduce uncertainties affecting density-derived porosities in the presence of variable, unknown organic matter content.
The first NMR log run in a silicilyte well could not be fully reconciled with density data. Laboratory NMR confirmed the log measurements, but more questions were raised when significant NMR signal was recorded on dry, cleaned samples at the time when new field data could not be explained.
Athel silicilyte is unique, and with no reference to confirm or to invalidate new data and interpretation, consistency and integration are key to improving our understanding of the rock. A thorough review of NMR acquisition procedures and processing algorithms was undertaken with logging contractors; job planning was improved, new logging procedures were implemented, tool malfunctions were identified and remedied, processing methods were upgraded. Laboratory procedures were revisited, and NMR core measurements were complemented with additional, independent core analyses.
Whereas the initial emphasis was on "effective" porosity determination, NMR logs and core measurements provided new insight into the Athel silicilyte rock. This includes:strong indications that the rock may be mixed-wet;encouraging results in the area of permeability estimation; andindeed higher confidence in the assessment of effective porosity.
A TOC (Total Organic Carbon) log can also be derived, which could find further application in source rock evaluation and the characterisation of organic-rich reservoirs. Integration of all available log data using a statistical approach now provides a consistent reservoir description, with limited core calibration requirements. Having successfully integrated NMR data to unravel the organic content of the rock, the improved petrophysical model provides the basis for sound reservoir characterisation, the foundation for better exploration, appraisal and development decisions.
Introduction
The Athel play has been the object of much exploration effort in Petroleum Development Oman (PDO). In 1997, an integrated asset team, the first in PDO, was established to demonstrate the commerciality of Al Noor, the first Athel discovery where six wells have now established 190.106 m3 oil in place. As plans to develop the field matured, the focus shifted from regional studies to reservoir characterisation. At the same time, the objectives of well evaluations evolved from a stand-alone assessment of average porosity, net-to-gross, and saturation towards the determination of the vertical and lateral distribution of reservoir properties. Accurate porosity evaluation is a key element in this context.
After briefly reviewing the foundations of the original porosity model established to evaluate Athel silicilyte wells, this paper shows how NMR is being used to improve the assessment of porosity from wireline logs. The details of the experience gained in the application of laboratory and wellbore NMR technology to Athel silicilyte are presented.
The Al Noor Discovery
The Al Noor accumulation is located in the South Oman Salt Basin. There, the 400-m thick Athel silicilyte is encased in salt at around 4200 m depth. Silicilyte is not a term used frequently outside of PDO or its contractors. However, it has been defined previously[1] as "… a sedimentary rock composed principally of siliceous remains of organisms".
The reservoir potential of this unique rock was not recognised until 1989, when the Al Noor-l exploration well proved the producibility of hydrocarbons from the Athel "source rock".
